﻿using System;
using System.IO;

namespace CuteAnt.Security
{
	/// <summary>CRC16校验</summary>
	public sealed class Crc16
	{
		#region 数据表

		/// <summary>CRC16表</summary>
		private static readonly UInt16[] CrcTable = new UInt16[256]
        {
            /* CRC16 余式表 */
            0x0000,0x1021,0x2042,0x3063,0x4084,0x50A5,0x60C6,0x70E7,
            0x8108,0x9129,0xA14A,0xB16B,0xC18C,0xD1AD,0xE1CE,0xF1EF,
            0x1231,0x0210,0x3273,0x2252,0x52B5,0x4294,0x72F7,0x62D6,
            0x9339,0x8318,0xB37B,0xA35A,0xD3BD,0xC39C,0xF3FF,0xE3DE,
            0x2462,0x3443,0x0420,0x1401,0x64E6,0x74C7,0x44A4,0x5485,
            0xA56A,0xB54B,0x8528,0x9509,0xE5EE,0xF5CF,0xC5AC,0xD58D,
            0x3653,0x2672,0x1611,0x0630,0x76D7,0x66F6,0x5695,0x46B4,
            0xB75B,0xA77A,0x9719,0x8738,0xF7DF,0xE7FE,0xD79D,0xC7BC,
            0x48C4,0x58E5,0x6886,0x78A7,0x0840,0x1861,0x2802,0x3823,
            0xC9CC,0xD9ED,0xE98E,0xF9AF,0x8948,0x9969,0xA90A,0xB92B,
            0x5AF5,0x4AD4,0x7AB7,0x6A96,0x1A71,0x0A50,0x3A33,0x2A12,
            0xDBFD,0xCBDC,0xFBBF,0xEB9E,0x9B79,0x8B58,0xBB3B,0xAB1A,
            0x6CA6,0x7C87,0x4CE4,0x5CC5,0x2C22,0x3C03,0x0C60,0x1C41,
            0xEDAE,0xFD8F,0xCDEC,0xDDCD,0xAD2A,0xBD0B,0x8D68,0x9D49,
            0x7E97,0x6EB6,0x5ED5,0x4EF4,0x3E13,0x2E32,0x1E51,0x0E70,
            0xFF9F,0xEFBE,0xDFDD,0xCFFC,0xBF1B,0xAF3A,0x9F59,0x8F78,
            0x9188,0x81A9,0xB1CA,0xA1EB,0xD10C,0xC12D,0xF14E,0xE16F,
            0x1080,0x00A1,0x30C2,0x20E3,0x5004,0x4025,0x7046,0x6067,
            0x83B9,0x9398,0xA3FB,0xB3DA,0xC33D,0xD31C,0xE37F,0xF35E,
            0x02B1,0x1290,0x22F3,0x32D2,0x4235,0x5214,0x6277,0x7256,
            0xB5EA,0xA5CB,0x95A8,0x8589,0xF56E,0xE54F,0xD52C,0xC50D,
            0x34E2,0x24C3,0x14A0,0x0481,0x7466,0x6447,0x5424,0x4405,
            0xA7DB,0xB7FA,0x8799,0x97B8,0xE75F,0xF77E,0xC71D,0xD73C,
            0x26D3,0x36F2,0x0691,0x16B0,0x6657,0x7676,0x4615,0x5634,
            0xD94C,0xC96D,0xF90E,0xE92F,0x99C8,0x89E9,0xB98A,0xA9AB,
            0x5844,0x4865,0x7806,0x6827,0x18C0,0x08E1,0x3882,0x28A3,
            0xCB7D,0xDB5C,0xEB3F,0xFB1E,0x8BF9,0x9BD8,0xABBB,0xBB9A,
            0x4A75,0x5A54,0x6A37,0x7A16,0x0AF1,0x1AD0,0x2AB3,0x3A92,
            0xFD2E,0xED0F,0xDD6C,0xCD4D,0xBDAA,0xAD8B,0x9DE8,0x8DC9,
            0x7C26,0x6C07,0x5C64,0x4C45,0x3CA2,0x2C83,0x1CE0,0x0CC1,
            0xEF1F,0xFF3E,0xCF5D,0xDF7C,0xAF9B,0xBFBA,0x8FD9,0x9FF8,
            0x6E17,0x7E36,0x4E55,0x5E74,0x2E93,0x3EB2,0x0ED1,0x1EF0
        };

		#endregion

		///// <summary>取得CRC16校验码</summary>
		///// <param name="pcrc"></param>
		///// <returns></returns>
		//public UInt16 CRC16Code(Byte[] pcrc)
		//{
		//    UInt16 crc16 = 0;
		//    for (Int32 i = 0; i < pcrc.Length; ++i)
		//    {
		//        crc16 = (UInt16)((crc16 << 8) ^ CrcTable[((crc16 >> 8) ^ pcrc[i])]);
		//    }
		//    return crc16;
		//}
		/// <summary>校验值</summary>
		private UInt16 crc = 0xFFFF;

		/// <summary>校验值</summary>
		public UInt16 Value { get { return crc; } set { crc = value; } }

		/// <summary>重置清零</summary>
		public Crc16 Reset()
		{
			crc = 0xFFFF; return this;
		}

		/// <summary>添加整数进行校验</summary>
		/// <param name = "value">
		/// the Byte is taken as the lower 8 bits of value
		/// </param>
		public Crc16 Update(Int16 value)
		{
			//crc = CrcTable[(crc ^ value) & 0xFF] ^ (crc >> 8);
			crc = (UInt16)((crc << 8) ^ CrcTable[((crc >> 8) ^ value)]);
			return this;
		}

		/// <summary>添加字节数组进行校验</summary>
		/// <param name = "buffer">
		/// The buffer which contains the data
		/// </param>
		/// <param name = "offset">
		/// The offset in the buffer where the data starts
		/// </param>
		/// <param name = "count">
		/// The number of data bytes to update the CRC with.
		/// </param>
		public Crc16 Update(Byte[] buffer, Int32 offset = 0, Int32 count = 0)
		{
			if (buffer == null) throw new ArgumentNullException("buffer");
			if (count < 0) throw new ArgumentOutOfRangeException("count", "Count不能小于0！");
			if (count == 0) count = buffer.Length;
			if (offset < 0 || offset + count > buffer.Length) throw new ArgumentOutOfRangeException("offset");

			//while (--count >= 0)
			//{
			//    crc = CrcTable[(crc ^ buffer[offset++]) & 0xFF] ^ (crc >> 8);
			//}
			//crc16 = (UInt16)((crc16 << 8) ^ CrcTable[((crc16 >> 8) ^ pcrc[i])]);
			crc ^= crc;

			for (Int32 i = 0; i < count; i++)
			{
				crc = (UInt16)((crc << 8) ^ CrcTable[(crc >> 8 ^ buffer[offset + i]) & 0xFF]);
			}
			crc ^= crc;
			return this;
		}

		/// <summary>添加数据流进行校验</summary>
		/// <param name="stream"></param>
		/// <param name="count">数量</param>
		public Crc16 Update(Stream stream, Int64 count = 0)
		{
			if (stream == null) throw new ArgumentNullException("stream");
			if (count < 0) throw new ArgumentOutOfRangeException("count", "Count不能小于0！");
			if (count == 0) count = Int64.MaxValue;

			while (--count >= 0)
			{
				var b = stream.ReadByte();
				if (b == -1) { break; }

				//crc = CrcTable[(crc ^ b) & 0xFF] ^ (crc >> 8);
				//crc = (UInt16)((crc << 8) ^ CrcTable[(crc ^ b) & 0xFF]);
				crc ^= (Byte)b;

				for (Int32 i = 0; i < 8; i++)
				{
					if ((crc & 0x0001) != 0)
						crc = (UInt16)((crc >> 1) ^ 0xa001);
					else
						crc = (UInt16)(crc >> 1);
				}
			}
			return this;
		}
	}
}